Still and all, one should not think that he cannot daven if he isn't feeling enthusiastic and fiery at the moment. That is not a reason to push off a tefillah. Rather, one should daven as well as one can at the time.
It is related that a man once came to the Bais Yisroel of Gur zy"a and sadly said that his tefillos were not coming out with as much emotion as he would like. He said, "Es davent zich nisht."
The Rebbe replied, "Look in Shulchan Aruch, Hilchos Tefillah. You won't find any halacha that one must be enthusiastic or emotional when he davens. All you will find is that it says that everyone must daven three tefillos every day and one must attempt to understand the simple meaning of the words. It doesn't say anything more than that."
(It is said in the name of Rav Shalom of Belz zy"a, "Just from davening and understanding the words, one can reach levels of ruach hakodesh and the revelation of Eliyahu Hanavi.")
The tzadikim of Lechovitch-Kobrin would say, "We daven one word and then another word, until it becomes a war." This is like two people who are arguing, with each stating his opinion, until it gets so heated that a real fight breaks out. So too, we say one word after another, getting more enthusiastic with each word, until we arouse real emotion.
